Conventionally, materials that block the light in the visible region and transmit the light in the infrared region find various applications. Although various materials such as color glass are conceivable, few candidates that have durability at relatively high temperatures (for example, greater than or equal to about 700° C. and less than or equal to about 1200° C. in vacuo or in an inert gas) and can bear high quantity of heat (or can bear rapid temperature change) are known. As a material capable of bearing light of high strength and sufficiently radiating heat, diamond materials are recited.
However, those having light transmissivity in the visible region as with the diamond materials are common, and they do not completely block the light in the visible region. If there is a material that blocks the light in the visible region and transmits the light in the infrared region, it can be used as a reference for temperature measurement of a heated object in vacuo as a visible region temperature monitoring chip of ideal black body radiation. If the material is insulative, it can be used also on an energized heating unit. Such a material can also be used as a window through which the interior of a dark room can be monitored with the infrared light. This means application to a window intended for observation of a nocturnal animal. This also allows measurement of the temperature in monitoring the temperature of the edge of a tool.
Diamond has a band gap of as large as 5.5 eV, and is generally believed to be transparent. However, diamond can be made to have absorption at a specific wavelength by means of impurities. As disclosed, for example, in Takashi TSUNO, Nobuhiro OTA, and Yoshiaki KUMAZAWA, Proceedings of 9th Diamond Symposium, p. 61 (NPD 1), Robert Linares, Patrick Doering, Diamond and Related Materials 8 (1999) p. 909 (NPD 2) and National Patent Publication No. 2005-512929 (PTD 1), it is known that light absorption occurs in the ultraviolet region around a wavelength of 270 nm when the diamond is doped with nitrogen in a substituted form as an impurity. Moreover, as disclosed in National Patent Publication No. 2005-512929 (PTD 1), it is known that absorption occurs in the wavelength region of around 500 nm to 640 nm due to association of nitrogen added to the diamond as an impurity with a vacancy defect. It is known that such absorption makes the diamond transparent yellow or transparent red.